Micro photoluminescence imaging with optical filtering

ABSTRACT

A method that includes: illuminating a wafer with excitation light having a wavelength and intensity sufficient to induce photoluminescence in the wafer; filtering photoluminescence emitted from a portion of the wafer in response to the illumination; directing the filtered photoluminescence onto a detector to image the portion of the wafer on the detector with a spatial resolution of 1 μm×1 μm or smaller; and identifying one or more crystallographic defects in the wafer based on the detected filtered photoluminescence.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. application Ser. No.14/703,692, entitled MICRO PHOTOLUMINESCENCE IMAGING, filed May 4, 2015.

TECHNICAL FIELD

This disclosure relates to identifying defects in integrated circuitdevices, such as complementary metal-oxide semiconductor imagingsensors.

BACKGROUND

Semiconductor materials are widely used in electronics andoptoelectronics. Crystalline semiconductor materials are prone tocrystallographic defects, which may be detrimental to the performance ofa device utilizing that material. Crystallographic defects may result inassociated photoluminescence which can be used to identify the defects.

An example of an optoelectronic device that utilizes crystallinesemiconductor materials is a complementary metal-oxide semiconductor(CMOS) imaging sensor (CIS) is an integrated circuit (IC) device. A CISdevice is used to convert a light intensity pattern into electricdigital signals. In some cases, a CIS is a two dimensional array ofphotodiodes with accompanying CMOS logic for signal processing. Eachindividual photodiode with processing CMOS logic is referred to a pixel.In some cases, a CIS has 1,000,000 or more pixels.

A CIS is commonly fabricated on n/n++ or p/p++ wafers. As an example, insome cases, thin lightly doped n-type or p-type epitaxial layers (e.g.,3-5 μm layers each having a dopant concentration of 1×10¹⁴ to 1×10¹⁵cm⁻³) are grown on a highly doped n++ or p++ substrate (e.g., asubstrate having a dopant concentration of 1×10¹⁸ to 1×10²⁰ cm⁻³). A CISis formed on the epitaxial layers, a region often referred to as thedevice active area. Performance of the CIS is influenced, at least inpart, by properties of this active area.

The highly doped substrates (often referred to as handles) providemechanical support for the active area during the CIS fabricationprocess. In some cases, the substrate also reduces the occurrence ofcross-talk in a CIS. For example, the substrate can reduce thecross-talk that results when minority carriers generated underneath onepixel in response to red light reach adjacent pixels of the CIS.

A CIS can be arranged according to a variety of differentconfigurations. For example, a CIS can be arranged as a front sideilluminated (FSI) CIS, or as a back side illuminated (BSI) CIS. Here,the “front” side refers to the side of the wafer on which the IC pixelstructures are fabricated. In some cases, to make a BSI CIS, a CIS waferfirst undergoes CIS processing on its front side. The CIS wafer is thenbonded along its front side to a wafer carrier, and its backside isthinned (e.g., by a few μm) until all of its n++ or p++ substrate isremoved. The surface of the CIS wafer is then passivated and coveredwith an antireflection coating, and color filters are fabricated on itsback side. During use, a light image is projected on the back side ofthe CIS wafer, and the CIS converts the light image into electricdigital signals.

Light from an image projected on a CIS having photon energy larger thanthe silicon band gap is primarily absorbed in the CIS active area. Thisabsorption generates electron and hole pairs, resulting in photocurrent.These photo-generated minority carriers are then collected by a p-njunction at this location. The number of photo-generated minoritycarriers is proportional to the number of photons that are absorbed inthe CIS active area, and varies according to the intensity of light.Thus, the intensity of light incident upon the CIS active area can bededuced based on the magnitude of the generated photocurrent. Inpractice, it is often desirable for each of the pixels of a CIS togenerate identical or substantially similar photocurrent in response touniform, low level illumination. Otherwise, pixels having lower orhigher photocurrent (e.g., “defective” pixels) might result in bright ordark spots in the resulting image (i.e., higher image intensity thandefect-free regions).

In some cases, localized crystallographic defects and heavy metalcontaminations could increase or decrease photocurrent from a givenpixel, resulting in an image having bright spots or dark spots at lowillumination levels. When present in a space charge region of the p-njunctions, these defects act as generation centers for minoritycarriers. This results in an increase in the dark current of thesepixels, and if the defect is sufficiently severe, will result in whiteor bright spots in the resulting image. When present outside of thespace charge region of p-n junctions, these defects act as recombinationcenters for minority carriers. This results in a decrease in the amountof photocurrent collected by the junctions, and if the defect issufficiently severe, will result in as dark spots in the resulting imageat low illumination levels.

Localized crystallographic defects or heavy metal contaminations canpotentially be introduced at any step during the fabrication process ofa CIS. Thus, to improve and control the fabrication process of a CIS, itis important to quickly identify processing steps that are introducingthese defects.

SUMMARY

Systems and techniques for identifying defects in semiconductormaterials, including those found in integrated circuit devices, aredescribed herein. More specifically, the systems and techniques areuseful for identifying localized individual crystallographic defects insemiconductor materials that exhibit photoluminescence, particularlywhere the defect PL is at different energies compared to theband-to-band PL. Such crystallographic defects may be introduced into asemiconductor material at various stages of material or devicefabrication, such as CIS fabrication.

In general, in a first aspect, the invention features a method thatincludes: illuminating a wafer with excitation light having a wavelengthand intensity sufficient to induce photoluminescence in the wafer;filtering photoluminescence emitted from a portion of the wafer inresponse to the illumination; directing the filtered photoluminescenceonto a detector to image the portion of the wafer on the detector with aspatial resolution of 1 μm×1 μm or smaller; and identifying one or morecrystallographic defects in the wafer based on the detected filteredphotoluminesence.

Implementations of the method may include one or more of the followingfeatures and/or features of other aspect. For example, the detectedfiltered photoluminescence can correspond to photoluminescence fromcrystallographic defects in the wafer. The filtering can substantiallyremove photoluminescence from band-to-band transitions in the wafer fromthe filtered photoluminescence.

The detected filtered photoluminescence can include light having anenergy in a range from about 0.7 eV to about 0.9 eV. The wafer can be asilicon wafer. The filtering can substantially block light having anenergy of more than about 1.0 eV from being detected.

In some implementations, the filtering further includes filteringexcitation light reflected from the portion of the wafer.

Some implementations further include detecting excitation lightreflected from the portion of the wafer. The method can also includecomparing the detected photoluminescence with the detected excitationlight and identifying one or more defects in the wafer based on thecomparison.

The wafer can be for a complementary metal-oxide semiconductor (CMOS)imaging sensor. The portion of the wafer can correspond to one or morepixels of the CMOS imaging sensor. Identifying one or more defects inthe wafer can include identifying one or more defective pixels of theCMOS imaging sensor. In some implementations, identifying one or moredefects in the wafer includes identifying one or more defects having adimension of 1 μm or smaller.

The method can further include forming a photoluminescence intensity mapof the portion of the wafer based on the photoluminescence emitted fromthe portion of the wafer; and forming a reflection intensity map of theportion of the wafer based on the excitation light reflected from theportion of the wafer. Comparing the detected photoluminescence from theportion of the wafer and the detected reflected excitation light fromthe region of the wafer can include: determining that thephotoluminescence intensity map includes a first variation in intensityat a first location of the wafer; upon determining that thephotoluminescence intensity map includes the first variation inintensity at the first location of the wafer, determining whether thereflection intensity map includes a second variation in intensity at thefirst location of the wafer; and upon determining that the reflectionintensity map does not include a second variation in intensity at thefirst location of the wafer, determining that a defect is present at thefirst location of the wafer. Comparing the detected photoluminescencefrom the portion of the wafer and the detected reflected excitationlight from the region of the wafer can further include: upon determiningthat the reflection intensity map includes the second variation inintensity at the first location of the wafer, determining that a defectis not present at the first location of the wafer.

In some implementations, the method further includes adjusting aproperty of the excitation light. Adjusting the property of theexcitation light can include adjusting a wavelength of the excitationlight. The wavelength of the excitation light can be adjusted toincrease photoluminescence emitted from the second portion of the wafer.The second portion of the wafer can be at a different depth in the waferfrom the first portion.

The excitation light can have a wavelength in a range from 200 nm to1,100 nm.

The wafer can be a silicon wafer or a compound semiconductor wafer.

The method can further include performing a processing step on thewafer. The processing step can be selected from the group consisting ofan ion implantation step, an annealing step, a layer deposition step, anoxidation step, and a polishing step. The processing step can beperformed after identifying the crystallographic defects. The method caninclude illuminating the processed wafer with excitation light andidentifying one or more additional defects in the processed wafer basedon photoluminescence from the processed wafer. The method can includecomparing the crystallographic defects identified in the wafer with theadditional defects.

The crystallographic defects correspond to bright portions in an imageof a portion of the wafer.

In general, in another aspect, the invention features a system thatincludes: an illumination module configured to illuminate a wafer withexcitation light having a wavelength and intensity sufficient to inducephotoluminescence in the wafer; a detection module configured to detectphotoluminescence emitted from a portion of the wafer in response to theillumination; imaging optics configured to image the portion of thewafer onto the detection module with a spatial resolution of 1 μm×1 μmor smaller; an optical filter arranged to filter photoluminescenceemitted from the portion of the wafer prior to detection by thedetection module; and a processing module configured to identify one ormore crystallographic defects in the wafer based on the detectedfiltered photoluminescence.

Embodiments of the system can include one or more of the followingfeatures and/or features of other aspects. For example, the opticalfilter can transmit light corresponding to photoluminescence fromcrystallographic defects in the wafer to the detection module. Theoptical filter can substantially block photoluminescence fromband-to-band transitions in the wafer from the detection module.

The optical filter can transmit light having an energy in a range fromabout 0.7 eV to about 1.0 eV to the detection module. The optical filtercan substantially block light having an energy of more than about 1.0 eVfrom the detection module.

In some embodiments, the optical filter blocks from the detection moduleat least some of the excitation light reflected from the wafer towardsthe detection module.

The excitation light can have a wavelength in a range from 200 nm to1,100 nm.

The illumination assembly can be arranged to illuminate the excitationlight to the wafer along an optical axis non-normal to an illuminatedsurface of the wafer. The illumination optics can have an optical axisnominally normal to the illuminated surface of the wafer.

Among other advantages, embodiments may be used to identify localizeddefects in a CIS device during the manufacturing process (e.g., duringor between intermediate steps of the manufacturing process of the CISdevice) and/or after the completion of the manufacturing process (e.g.,as a part of a post-manufacturing inspection). In some cases,embodiments can be used to identify defects associated with a singlepixel of the CIS device, such that one or more individual defectivepixels can be identified in a CIS device. In some cases, embodiments canbe used to reduce the number of positives that might otherwise resultduring the defect detection process due to the presence of particulatematter on a CIS device. In certain embodiments, photoluminescence thatresults from crystallographic defects in a CIS device may bedistinguished from other sources of photoluminescence. For instance,optical filtering may be used to distinguish between photoluminescenceat different energies. Where photoluminescence from different sources(e.g., from crystallographic defects versus band to bandphotoluminescence), optical filtering can be used to isolate thephotoluminescence from the crystallographic defects and thereby locateindividual crystallographic defects in the device.

In general, bright field or dark field imaging may be used.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features and advantages willbe apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 shows an example system for detecting defects in a CIS sample.

FIG. 2 shows a photoluminescence intensity map having variations inphotoluminescence intensity characteristic of defects in a siliconsubstrate.

FIG. 3 shows another example system for detecting defects in a sample.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Defects in a device formed from a semiconductor material (e.g., a CISdevice) can be identified by inducing photoluminescence in the activeregion of the device, and examining the photoluminescence for localizedvariations in intensity.

For example, photoluminescence can be induced in a silicon wafer withlight having photon energy larger than the energy gap of silicon (e.g.,more than 1.1 eV). As this light is absorbed in the silicon,electron-hole pairs are generated in the silicon. Some of thesephoto-generated carriers will recombine through radiative recombinationand release photons of light, a phenomenon known as photoluminescence.

As the intensity of photoluminescence varies depending on thecomposition of the wafer, localized variation in the composition of thewafer (e.g., resulting from material defects or contaminations) willresult in localized variation in the induced photoluminescence. Thus,defects in a CIS device, for example, can be identified, at least inpart, by illuminating a CIS device with excitation light sufficient toinduce photoluminescence, and examining the photoluminescence forlocalized variations in intensity. While the following descriptionrefers to CIS defect evaluation, it will be understood that thedisclosed techniques can be applied more broadly to other devices usinga crystallographic semiconductor material that exhibits defectphotoluminescence.

An example system 100 for identifying defects in a CIS is shown inFIG. 1. The system 100 includes a stage assembly 110, an illuminationassembly 130, an optical assembly 150, and an imaging assembly 170. Inan example usage of system 100, a CIS sample 190 is placed on the stageassembly 110 and is positioned for examination. The illuminationassembly 130 generates excitation light suitable for inducingphotoluminescence in the CIS sample 190. The optical assembly 150directs the excitation light generated by the illumination assembly 130onto the CIS sample 190, thereby inducing photoluminescence in the CISsample 190 and/or causing excitation light to be reflected by the CISsample 190. The optical assembly directs the photoluminescence generatedby the CIS sample 190 and/or light reflected by the CIS sample 190towards the imaging assembly 170. The imaging assembly 170 detects thephotoluminescence and reflected excitation light, and identifies defectsin the CIS sample 190 based on the detected light.

The stage assembly 110 supports the CIS sample 190 during examination bythe system 100. In some cases, the stage assembly 110 can move along oneor more axes, such that the CIS sample 190 can be moved related to theillumination assembly 130, optical assembly 150, and/or the imagingassembly 170. For example, in some cases, the CIS can move along the x,y, and z axes of a Cartesian coordinate system in order to move the CISsample 190 along any of three dimensions related to the other componentsof the system 100.

The illumination assembly 130 generates excitation light that, whenincident upon the CIS sample 190, induces photoluminescence in the CISsample 190. The illumination assembly 130 includes light sources 132a-b, collimating lenses 134 a-b, filters 136 a-b, dichroic beam splitter138, and focusing lens 140.

Light sources 132 a-b generate light having particular propertiessuitable for inducing photoluminescence in the CIS sample 190. In somecases, the light sources 102 a-b are laser light sources that generatelight having a particular wavelength and intensity. In some cases, thelight sources 132 a-b each generate light having different wavelengths,such that the illumination assembly 130 can provide different types oflight. For example, the light source 132 a can generate light having afirst wavelength (e.g., 532 nm), and the light source 132 b can generatelight having a second wavelength (e.g., 880 nm).

As another example, either or both of the light sources 132 a-b cangenerate light having a wavelength less than 532 nm (e.g., 300 nm, 350nm, 400 nm, 450 nm, 500 nm, or any intermediate wavelength thereof). Asyet another example, either or both of the light sources 132 a-b cangenerate light having a wavelength between 200 nm and 1100 nm (e.g., 200nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm,1100 nm, or any intermediate wavelength thereof). Although examplewavelengths are described above, these are merely illustrative examples.In practice, the light sources 132 a-b can each generate light havingany other wavelengths, depending on the implementation.

The light sources 132 a-b can be operated independently from oneanother, such that light at each of the different wavelengths can beindividually or simultaneously generated. Light sources 132 a-b includeany component capable to generating light at a specified wavelength. Forexample, in some cases, the light sources 132 a-b can include one ormore lasers or light emitting diodes (LEDs).

The light generated by the light sources 132 a-b can also vary inintensity, depending on the implementation. As an example, in somecases, the light sources 132 a-b can each generate light having a powerbetween 0.02 W and 20 W. In some cases, the intensity of light generatedby the light sources 132 a-b can also be adjusted during use of thesystem 100. For example, in some cases, the light generated by the lightsources 132 a-b can be adjusted between 0.02 W and 20 W during operationof the system 100. As another example, in some cases, the lightgenerated by the light sources 132 a-b can be adjusted during operationof the system 100 such that they generate light having a power less than0.02 W (e.g., 0.015 W, 0.010 W, or 0.005 W). Although exampleintensities are described above, these are merely illustrative examples.In practice, the light sources 132 a-b can each generate light havingother intensities, depending on the implementation.

Excitation light generated by the light sources 132 a-b are directedtowards collimating lenses 134 a-b, respectively. The collimating lenses134 a-b narrows the beam of passing light, such that the light exitingthe collimating lenses 134 a-b are aligned along the optical axes ofcollimating lenses 134 a-b, respectively.

The collimated excitation light from collimating lenses 134 a-b aredirected into filters 136 a-b, respectively. The filters 136 a-b filterthe passing light, such that only light having particular wavelengths(or correspondingly, energies) or range of wavelengths are substantiallytransmitted through the filters 136 a-b, respectively. The filters 136a-b can be used to “clean” the light generated by the light sources 132a-b. For example, if the light source 132 a generates light having afirst wavelength (e.g., 532 nm), the filter 136 a can be a band-passfilter that transmits light having a range of wavelengths that includesthe first wavelength (e.g., 522 nm to 542 nm), while light havingwavelengths outside of this range are not substantially transmitted. Asanother example, if the light source 132 b generates light having afirst wavelength (e.g., 880 nm), the filter 136 b can be a band-passfilter that transmits light having a range of wavelengths that includesthe second wavelength (e.g., 870 nm to 890 nm), while light havingwavelengths outside of this range are not substantially transmitted. Insome cases, for example in implementations in which the light sources132 a-b include one or more lasers, the filters 136 a-b can also includespeckle-reducing elements (e.g., a moving diffuser element) in order toreduce the effects of interference effects in the laser beam.

The filtered excitation light from the filters 136 a-b are directed intoa dichroic beam splitter 138. The dichroic beam splitter 138 reflectslight and/or transmits light, depending on the wavelength of lightincident upon it. For example, if the light source 132 a generates lighthaving a first wavelength (e.g., 532 nm) and the light source 132 bgenerates light having a second wavelength (e.g., 880 nm), the dichroicbeam splitter 138 can transmit light having the first wavelength andreflects light having the second wavelength. As a result, although lightgenerated by each of the light sources 132 a-b are initially directed insubstantially different directions, dichroic beam splitter 138 redirectsthe light in a substantially similar direction.

The excitation light from the dichroic beam splitter 138 is directed toa focusing lens 140. The focusing lens 140 focuses the light towards theoptical assembly 150.

The optical assembly 150 directs the excitation light generated by theillumination assembly 130 towards the CIS sample 190, and directsphotoluminescence generated by the CIS sample 190 and/or light reflectedby the CIS sample 190 towards the imaging assembly 170. The opticalassembly 150 includes dichroic beam splitters 152 and 156, an objectivelens 154, a filter 158, and field lenses 160 a-b.

The excitation light from the focusing lens 140 is directed to thedichroic beam splitter 152. The dichroic beam splitter 152 reflectslight and/or transmits light, depending on the wavelength of lightincident upon it. For example, if the light source 130 a generatesexcitation light having a first wavelength (e.g., 532 nm), the lightsource 130 b generates excitation light having a second wavelength(e.g., 880 nm), and photoluminescence induced in the CIS sample 190 hasa third wavelength (e.g., 1100 nm), dichroic beam splitter 152 canpartially reflect and partially transmit the light at each of thesewavelengths. Thus, at least some of the excitation light received fromthe illumination assembly 130 is redirected by the dichroic beamsplitter 152 towards the objective lens 154, at least some of thephotoluminescence induced in the CIS sample 190 is transmitted by thedichroic beam splitter 152 towards the imaging assembly 170, and atleast some of the excitation light reflected by the CIS sample 190 isalso transmitted towards the imaging assembly 170.

The excitation light from the dichroic beam splitter 152 is directed toan objective lens assembly 154. The objective lens assembly 154 directsthe excitation light onto the CIS sample 190. In some cases, theobjective lens assembly 154 can direct the excitation light onto aparticular region of the CIS sample 190 (e.g., a region of the CISsample 190 that is being examined), such that the intensity ofexcitation light incident upon that region of the CIS sample 190 isuniform or substantially uniform. This region can be, for example, theentirety of the CIS sample 190 or a portion of the CIS sample 190.

The excitation light incident on the CIS sample 190 can inducephotoluminescence in the CIS sample 190. In some cases, thephotoluminescence in the CIS sample 190 can have a wavelength between950 nm and 1800 nm (e.g., between 1100 nm and 1550 nm).

The objective lens assembly 154 can focus on particular regions of theCIS sample 190 in order to obtain photoluminescence from these regions.In some cases, the objective lens assembly 154 can focus on a region ofthe CIS sample 190 that includes one or more pixels of the CIS sample190, and the objective lens assembly 154 can include a lens elementhaving a wide angle and shallow depth of field, such that it resolvesphotoluminescence from each of the pixels within that region. In somecases, the objective lens assembly have a lens element having a focallength between 0.5 mm and 550 mm, and a depth of field between 1 μm and400 μm. In some cases, the objective lens assembly 154 can resolve lightwith sufficient resolution to distinguish photoluminescence from each ofthe pixels. For example, if the CIS sample 190 includes pixels havingdimensions of 1 μm×1 μm along the surface, the objective lens assembly154 can resolve photoluminescence at a spatial resolution of 1 μm×1 μmor finer.

The excitation light incident on the CIS sample 190 can also result inthe reflection of excitation light from the CIS sample 190. Theobjective lens assembly 154 can also focus on particular regions of theCIS sample 190 in order to obtain excitation light reflected from theseregions. In an similar manner as above, in some cases, the objectivelens assembly 154 can focus on a region of the CIS sample 190 thatincludes one or more pixels of the CIS sample 190, and the objectivelens assembly 154 can include a lens element having a wide angle andshallow depth of field, such that it resolves excitation light reflectedfrom each of the pixels within that region. In a similar manner asabove, some cases, the objective lens assembly 154 can resolve lightwith sufficient resolution to distinguish excited light reflected fromeach of the pixels. For example, if the CIS sample 190 includes pixelshaving dimensions of 1 μm×1 μm along the surface, the objective lensassembly 154 can resolve reflected light at a spatial resolution of 1μm×1 μm or finer.

In some cases, the objective lens assembly 154 can be refocused in orderto resolve light from different regions of the CIS sample 190. Forexample, in some implementations, the focal depth of objective lensassembly 154 can be varied in order to examine photoluminescence fromvarying depths from the surface of the CIS sample 190 (e.g., from theback surface of the CIS sample 190 to the front surface of the CISsample 190).

In some cases, the magnification of the objective lens assembly 154 alsocan be changed in order to examine particular regions of the CIS sample190 in greater or lesser detail. In some cases, the magnitude of theobjective lens assembly 154 can be changed by moving lens elements ofthe objective lens assembly 154 relative to one another (e.g., a “zoom”lens), or by otherwise modifying the light path of light through theobjective lens assembly 154.

The photoluminescence and reflected excitation light is directed by theobjective lens assembly 154 to the dichroic beam splitter 152. Asdescribed above, the dichroic beam splitter 152 reflects light and/ortransmits light, depending on the wavelength of light incident upon it.For example, if the light source 130 a generates excitation light havinga first wavelength (e.g., 532 nm), the light source 130 b generatesexcitation light having a second wavelength (e.g., 880 nm), andphotoluminescence induced in the CIS sample 190 has a third wavelength(e.g., 1100 nm), dichroic beam splitter 152 can partially reflect andpartially transmit the light at each of these wavelengths. Thus, atleast some of the photoluminescence and reflected excitation light istransmitted by the dichroic beam splitter 152 towards the imagingassembly 170.

At least a portion of the photoluminescence and reflected excitationlight is directed by the dichroic beam splitter 152 to the dichroic beamsplitter 156. The dichroic beam splitter 156 also reflects light and/ortransmits light, depending on the wavelength of light incident upon it.For example, if the light source 130 a generates excitation light havinga first wavelength (e.g., 532 nm), the light source 130 b generatesexcitation light having a second wavelength (e.g., 880 nm), andphotoluminescence induced in the CIS sample 190 has a third wavelength(e.g., 1100 nm), dichroic beam splitter 156 can reflect excitation lighthaving the first and second wavelengths, and transmit photoluminescencehaving the third wavelength. As a result, photoluminescence andreflected excitation light are redirected along different optical paths.

Photoluminescence transmitted by the dichroic beam splitter 152 isdirected through a filter 158. The filter 158 filters the passing light,such that only light having particular wavelengths or range ofwavelengths are substantially transmitted through the filter 158. Insome embodiments, the filter 158 can be used to “clean” the output ofthe dichroic beam splitter 152. For example, if photoluminescence fromthe CIS sample 190 is expected to have a particular wavelength (e.g.,1100 nm), the filter 158 can be a band-pass filter that transmits lighthaving a range of wavelengths that includes the photoluminescencewavelength (e.g., 1000 nm to 1200 nm), while light having wavelengthsoutside of this range are not substantially transmitted. As anotherexample, in some cases, the filter 158 can be a long-pass filter thatattenuates light having relatively shorter wavelengths, whiletransmitting light having relatively longer wavelengths. This can behelpful, for example, in filtering out reflected excitation light, whichin many cases has a shorter wavelength than the photoluminescence fromthe CIS sample 190. Photoluminescence and the reflected excitation lightare then directed to field lenses 160 a-b respectively. The field lenses160 a-b focus the photoluminescence and the reflected excitation lighttowards the detectors 172 a-b, respectively, of the optical assembly170.

The imaging assembly 170 detects the photoluminescence and reflectedexcitation light, and identifies defects in the CIS sample based 190 onthe detected light. The imaging assembly 170 includes detectors 172 a-b,and a processing module 174.

The detectors 172 a-b measure the photoluminescence and the reflectedexcitation light, respectively, from the dichroic beam splitter 152. Insome cases, the detectors 172 a-b are configured to measure theintensity of light at a sufficiently high spatial resolution to resolvephotoluminescence and the reflected excitation light for a single pixelof the CIS sample 190. For example, if the CIS sample 190 includespixels having dimensions of 1 μm×1 μm along the surface, the detectors172 a-b can each resolve photoluminescence at a spatial resolution of 1μm×1 μm or finer. In some cases, the detectors 172 a-b can include asingle detection element that measures the intensity of light incidentupon it, or several such detection elements. For example, in some cases,the detectors 172 a-b can include a line of detection elements (e.g., a“line” detector), or a two dimensional array of detection elements. Insome cases, the detectors 172 a-b can be include one or more InGasAsline cameras or arrays, or Si line cameras or arrays.

In some cases, the detectors 172 a-b measure the photoluminescence andthe reflected excitation light, respectively, by integrating theintensity of light received over a period of time. This integration timecan depend, at least in the part, on the intensity of light that isapplied to the CIS sample 190. For example, in some cases, reducing theintensity of light incident on the CIS sample 190 by a factor of two canresult in an increase in the integration time by a factor of two. Asmeasurement noise from the detector increases with integration time, insome cases, the intensity of light that is applied to the CIS sample 190can be adjusted in order to limit the resulting measurement noise of thedetectors 172 a-b to appropriate levels. In some cases, the detectors172 a-b can be cooled in order to further reduce measurement noise. Forexample, in some cases, either or both of the detectors 172 a-b can becooled (e.g., by a Peltier cooler) to a particular temperature (e.g.,100 K) in order to reduce the amount of noise in the resultingmeasurements.

Measurements from the detectors 172 a-b are transmitted to theprocessing module 174 for interpretation. In some cases, processingmodule 174 can generate one or more multiply dimensional maps thatrepresent the intensity of photoluminescence and reflected excitationlight for a particular portion of the CIS sample 190. For example, insome cases, the detectors 172 a-b can include a two dimensional array ofdetection elements that each measure the intensity of light incidentupon that detection element. Using this information, the processingmodule 174 can generate spatial maps that represent the intensity ofphotoluminescence and reflected excitation light for at specificlocations on the CIS sample 190.

The processing module 174 can also identify defects in the CIS sample190 based on the measurements from the detectors 172 a-b. For example,the processing module 174 can identify regions of the CIS sample 190with a localized variation in photoluminescence (e.g., a spot, blotch,line, curve, or other region having photoluminescence that more intenseor less intense than the surrounding regions). The processing module 174can identify these regions of the CIS sample 190 as having defects. Insome cases, the processing module 174 can identify one or more specificpixels of the CIS sample 190 as being defective (e.g., pixels associatedwith the localized variation in photoluminescence).

In some cases, localized variation in photoluminescence might not be theresult of defects in the CIS sample, but rather might be the result ofparticulate matter on the surface of the CIS sample. As particulatematter can block or otherwise attenuate light, the presence of theseparticulates can locally affect the intensity of excitation lightincident on the CIS sample, and can result in localized variation inphotoluminescence. To distinguish between localized variations inphotoluminescence as a result of defects in the CIS sample from those asa result of particulate matter, the processing module 174 can determineif regions of the CIS sample 190 identified as having localizedvariations in photoluminescence also have corresponding localizedvariations in reflected excitation light.

As an example, if a region has both a localized variation inphotoluminescence and a corresponding localized variation in reflectedlight, the processing module 174 determines that the variation inphotoluminescence is the result of particulate matter on the surface ofthe CIS sample, and not a defect or contamination in the CIS sample.Thus, the processing module 174 can determine that no defect exists inthis region.

As another example, if the region has a localized variation inphotoluminescence, but does not have a corresponding localized variationin reflected light, the processing module 174 determines that thevariation in photoluminescence is not the result of particulate matteron the surface of the CIS sample. Thus, the processing module 174 candetermine that a defect exists in this region.

In some implementations, the processing module 174 can be implementedusing digital electronic circuitry, or in computer software, firmware,or hardware, or in combinations of one or more of them. For example, insome cases, the processing module 174 can be implemented, at least inpart, as one or more computer programs (e.g., one or more modules ofcomputer program instructions, encoded on computer storage medium forexecution by, or to control the operation of, a data processingapparatus). A computer storage medium can be, or can be included in, acomputer-readable storage device, a computer-readable storage substrate,a random or serial access memory array or device, or a combination ofone or more of them. The term “processing apparatus” encompasses allkinds of apparatus, devices, and machines for processing data, includingby way of example a programmable processor, a computer, a system on achip, or multiple ones, or combinations, of the foregoing. The apparatuscan include special purpose logic circuitry, e.g., an FPGA (fieldprogrammable gate array) or an ASIC (application specific integratedcircuit). The apparatus can also include, in addition to hardware, codethat creates an execution environment for the computer program inquestion, e.g., code that constitutes processor firmware, a protocolstack, a database management system, an operating system, across-platform runtime environment, a virtual machine, or a combinationof one or more of them. The apparatus and execution environment canrealize various different computing model infrastructures, such as webservices, distributed computing and grid computing infrastructures.

Although an example system 100 is shown and described, this is merely anillustrative example. In practice, the system 100 can have otherarrangements, depending on the implementation.

Implementations of the system 100 can be used to identify localizeddefects in a CIS device during the manufacturing process of the CISdevice (e.g., before, during, or after any step during the manufacturingprocess), and/or after the completion of the manufacturing process. Forexample, in some cases, implementations of the system 100 can be used tomonitor one or more intermediate steps in the manufacturing process ofone or more CIS devices, and/or to inspect one or more completed CISdevices.

In some cases, when defects are detected in a CIS device, informationregarding the location and nature of the defects can be used to modifythe manufacturing process such that fewer and/or less severe defects areintroduced into CIS devices in the future. For example, informationregarding the detection of defects can be used to identify particularmanufacturing equipment or processes that are partially or whollyresponsible for the defects. This information can then be used to repairand/or replace that equipment, or to modify the processes in order toimprove the manufacturing process. In some cases, information regardingthe location and nature of the defects can also be used to identifywafers or portions of wafers that are defective, such that these wafersor portions of wafers can be discarded or otherwise not used in futureprocesses.

Implementations of the system 100 can be used to identify localizeddefects in a CIS device with a spatial resolution of at least a singlepixel of the CIS device. For example, in some cases, a CIS device buildwith 32 nm technology has pixels of approximately 0.9×0.9 μm;implementations of the system 100 can be used to identify localizeddefects in this CIS device with a spatial resolution of 0.9×0.9 μm orfiner.

In general, the system 100 can generate various different types ofexcitation light, depending on the application. For instance, in somecases, the system 100 can vary the wavelength of excitation lightgenerated by the illumination module 130 in order to probe differentdepths under the surface of the CIS sample 190. As an example, in somecases, the illumination module 130 can generate green light (e.g.,having a wavelength of approximately 532 or 540 nm) in order to generateminority carriers and photoluminescence close to the surface of the CISsample 190 (e.g., where 1/(absorption coefficient)=1.5 μm). On the otherhand, in some cases, the illumination module 130 can generate nearinfrared illumination (e.g., having a wavelength of approximately 880nm) in order to generate minority carriers and photoluminescence furtheraway from the silicon surface. As described above, the illuminationmodule 130 can include multiple light sources, and each light source canbe selectively activated in order to product light having a differentwavelength. For instance, in the example system 100 shown in FIG. 1, theillumination module 130 can include two light sources, 132 a-b, eachconfigured to product light having a different wavelength. The lightsources 132 a-b can thus be selectively switching on or off in order toprobe different depths below the surface of the CIS sample 190. Althoughtwo light sources 132 a-b are shown, in practice, a system 100 caninclude any number of light sources, depending on the implementation.

In some cases, the system 100 can vary the intensity of excitation lightgenerated by the illumination module 130. In some cases, the excitationlight generated by the illumination module 130 can have an intensitythat is sufficiently high to induce photoluminescence in the CIS sample190, and also sufficiently low to such that Auger recombination does notsubstantially occur in the illuminated portion of the CIS sample 190.

In general, for a low injection level (e.g., when the minority carrierconcentration is less than the majority carrier concentration), theintensity of band-to-band photoluminescence is proportion to a productof the minority carrier concentration and the majority carrierconcentration at a particular location. For example, this can beexpressed as:PL=A*C _(minority carrier) *C _(majority carrier),where PL is the intensity of induced photoluminescence at a particularlocation (expressed as a number of photons), C_(minority carrier) is theminority carrier concentration at that location, C_(majority carrier) isthe majority carrier concentration at that location, and A is aconstant.

The minority carrier concentration C_(minority carrier) (referred to asan injection level) is proportional to an effective life time ofminority carriers and a generation rate (i.e., the number of photonsadsorbed in the silicon normalized to the volume of silicon whereminority carriers are present). For example, this can be expressed as:C _(minority carrier) =R _(generation) *t _(life,effective),where R_(generation) is the generation rate at the location andt_(life,effective) is the effective life time of minority carriers atthe location.

The intensity of induced photoluminescence at a particular location PLis proportional to the intensity of photoluminescence-inducing lightabsorbed in the silicon at that location (expressed as a number ofphotons), the effective life time of minority carriers at that location,and the dopant concentration in the silicon at that location. Forexample, this can be expressed as:PL=A*I _(absorbed) *t _(life,effective) *C _(majority carrier),where I_(absorbed) is the intensity of photoluminescence-inducing lightabsorbed in the silicon at the location (expressed as a number ofphotons).

The effective life time t_(life,effective) has contributions fromvarious recombination channels, in particular the bulk recombination,the recombination at the interfaces, and the Auger recombination. Forexample, this can be expressed as:1/t _(life,effective)=1/t _(recombination,bulk)+1/t_(recombination,interfaces)+1/t _(recombination,Auger),where t_(recombination,bulk) is the bulk recombination life time,t_(recombination,interfaces) is the interfaces recombination life time,and t_(recombination,Auger) is the Auger recombination life time.

Defects at interfaces of a pixel (e.g., at the front surface, the backsurface, or the walls of the deep trench insulation (DTI) reduces theeffective life time t_(life,effective) in a given pixel, and will causea reduction of photoluminescence intensity from this pixel. For example,this can be expressed as:1/t _(life,interfaces)=1/t _(recombination,front)+1/t_(recombination,back)+1/t _(recombination,DTI),where t_(recombination,front) is the front surface recombination lifetime, t_(recombination,back) is the back surface recombination lifetime, and t_(recombination,DTI) is the Deep Trench Isolation (DTI) wallrecombination life time.

The interface recombination life time t_(life,interfaces) is inverselyproportional to surface (i.e., interface) recombination velocity at thisinterface and the distance between these interfaces. For example, thiscan be expressed as:

${t_{{life},{interfaces}} = {\frac{1}{2}*\frac{d_{interfaces}}{v_{{recombination},{interface}}}}},$where d_(interfaces) is the distance between interfaces, andv_(recombination,interface) is the recombination velocity at theinterface.

In some cases, for the DTI interfaces, the distant between interfacesd_(interfaces) can be approximately 1 μm (i.e., for a pixel having adimension of 1 μm). The interfacial recombination rate for awell-passivated interface is in the range of 1 to 10 cm/sec. Assuming aninterfacial recombination rate of 10 cm/sec, one can expect that the DTIinterfacial recombination life time t_(recombination,DTI) to be about5×10⁻⁶ sec. This will control the effective life time t_(life,effective)in a pixel if there are no bulk defects and the surfaces are wellpassivated.

The effective life time t_(life,effective) depends on the injectionlevel (e.g., the intensity of light incident on the CIS active area).Thus, the effective life time t_(life,effective) can be controlled, atleast in part, by adjusting the injection level (e.g., by adjusting theintensity of the excitation light illuminating the CIS sample). Theinjection level can be adjusted according to one or more criteria.

For example, in some cases, the injection level can be adjusted in orderto reduce Auger recombination contribution to the effective life time inthe CIS active area. At high injection levels, the effective life timecan be controlled by Auger recombination. For example, in some cases,for a 1×10¹⁷ cm⁻³ injection level, the Auger recombination limits theeffective life time in p-type silicon to 1×10⁻⁴ seconds. As anotherexample, in some cases, for a 1×10¹⁸ cm⁻³ injection level, the Augerrecombination limits the effective life time in p-type silicon to 1×10⁻⁶seconds. Since Auger recombination is not sensitive to defects, theinjection level can be adjusted such that Auger recombination in the CISactive area does not dominate the recombination processes.

Further, Auger recombination also controls the effective life time inthe highly doped substrate. For instance, for p++ and n++ substrates,Auger recombination can limit the effective life time for all injectionlevels. As an example, in some cases, in a p++ substrate having a 1×10²⁰cm⁻³ carrier concentration, the effective life time is limited to 1×10⁻⁹sec. The Auger recombination life time changes abruptly with dopantconcentration. For example, a ten times reduction of the dopantconcentration (e.g., from a dopant concentration of 1×10²⁰ to 1×10¹⁹)will increase the effective life time one hundred times (e.g., to 1×10⁻⁷sec).

The effective life time in the CIS active area also depends on theinjection level at a low injection level regime (e.g., when thecontribution of Auger recombination is negligible). Thus, it isimportant to monitor photoluminescence intensity as a function of theillumination level (i.e., injection level), since the life time responseto the injection level could be different for various defects. Forexample, given a p-type silicon, a defect such as interstitial Fe willresult in an increase in the effective life time as the injection levelis increased. However, a defect such as a Fe—B pair will result in adecrease in the effective life time with an increase of injection level.For a defect which increases recombination at the interface (e.g., SiO₂interfaces or walls of the DTI), the effect of the changing injectionlevel on the interfacial recombination life time will depend on a stateof a space charge region at this interface. For the interface in aninversion at low injection levels, the interface recombination lifetimedoes not change with an increasing injection level. When the injectionlevel becomes high (e.g., larger than the majority carrierconcentration), then the life time decreases with the increasinginjection level. For the interface in a depletion for the low injectionlevels, the interfacial recombination life time increases with theincreasing injection level. But, for high injection levels, theinterfacial recombination life time does not change with an increase ininjection level. Therefore, for a given defective pixel, the dependenceof the photoluminescence intensity on the injection level could providean important clue to a nature of this defect.

As discussed above, in some cases, it is important to containphotoluminescence to the CIS active area and use appropriately lowinjection levels. Further, it is important to minimize or otherwiseappropriate reduce the amount of light that is absorbed in the highlydoped substrates. Photoluminescence intensity is proportional to themajority carrier concentration. Thus, for the same amount of photonsabsorbed in the substrate and the epitaxial layer (e.g., the CIS activearea), photoluminescence from the substrate could be stronger than fromthe CIS active region. For instance, in an example CIS device, the CISactive region has an average dopant concentration of 1×10¹⁶ cm⁻³ at thebeginning of processing, and has an effective life time of 5×10⁻⁶ sec.The substrate in this example has an average dopant concentration of1×10¹⁹ cm⁻³, and has an effective life time of 1×10⁻⁷ (controlled by theAuger recombination) which, in some cases, can result in a diffusionlength (e.g., the distance that minority carriers will diffuse) ofapproximately 20 μm. Given similar amounts of absorbed photons in theCIS active region and the highly doped substrate, the photoluminescencefrom the substrate will be five times more intense than for the CISactive area. To minimize the background photoluminescence from thesubstrate to less than 5% of the photoluminescence from the CIS activearea, the amount of photons absorbed in the substrate can be limited toless than 1% of the photons absorbed in the CIS active area. Therefore,the wavelength of the photoluminescence generating light (e.g.,absorption coefficient) can be chosen appropriately.

As described above, in some cases, it is also important to inducephotoluminescence using an injection level at which the Augerrecombination does not control the life time in the active area of theCIS sample (e.g., when the contribution of Auger recombination in theCIS active area is negligible). As discussed above, for an injectionlevel of 1×10¹⁷ cm⁻³, the Auger life time is 1×10⁻⁴ sec. But, for aninjection level of 1×10¹⁸ cm⁻³, the Auger life time is 1×10⁻⁶. As anexample, in some cases, for an CIS active area with an effective (i.e.,bulk and interfacial recombination) life time in the range of 5×10⁻⁶,the injection level of 1×10¹⁸ cm⁻³ can be avoided, as the sensitivity tothe bulk and the interfacial recombination would be lost at thisinjection level, and will contribute less than 20% to the measuredeffective lifetime.

In some cases, it is also important to induce photoluminescence using aninjection level that will prevent out diffusion of minority carriersfrom the CIS active area to the highly doped substrate. An importantconcern regarding the injection level (e.g., the minority carrierconcentration generated in the CIS active region) is related to outdiffusion of minority carriers generated in the CIS active area into thehighly doped substrate. Due to the dopant concentration differencebetween the highly doped p++ substrate and the lighter doped p-typematerials in the CIS active area, an electric field exists at the p/p++interface which, for low injection levels, will impede diffusion of theminority carriers from the CIS active area into the substrate. However,the difference in the minority carrier concentrations in the CIS activeregion and the substrate will generate a diffusion field which will bethe driving force for the minority carrier diffusion from the CIS activeregion into the substrate. As the injection level increases, thisdiffusion driving force will also increase. As long as the diffusionforce is smaller than the electric repulsion, the minority carriers willbe contained in the epitaxial layer. For high injection levels, thisdiffusion gradient can overcome the electric repulsion and some minoritycarriers could enter into the substrate. As a result, a strongbackground photoluminescence will be generated from the substrate. Thiswill reduce the sensitivity of detecting photoluminescence changes fromthe CIS active area.

Therefore, in order to enhance the detection sensitivity of the system100 and to minimize photoluminescence generation in the highly dopedsubstrate of a CIS sample, it is important to select an appropriatewavelength and intensity for the excitation light that is applied to theCIS sample.

In some cases, the wavelength and intensity of the excitation light canevaluated empirically for each application. For example, the empiricaldetermination can be made regarding whether the wavelength and intensityof the excitation light results in photoluminescence that ispredominantly from the active area of the CIS sample, or whether theresulting photoluminescence has a large contribution from the substrateof the CIS sample.

In an example evaluation process, the CIS sample is illuminated withexcitation light having a particular wavelength and intensity, and theresulting photoluminescence is detected over a relatively large portionof the CIS sample (e.g., a “macro” region). In some cases, the portionof the CIS sample that is examined in this manner can be larger than theportions of the CIS sample measured by the detectors 172 a-b, asdescribed above. In some cases, this “macro” region can have an area ofapproximately 1 cm² or larger (e.g., 1 cm², 2 cm², 3 cm², 4 cm² orlarger). In some cases, this “macro” region can include the entire CISsample.

In some cases, the photoluminescence of this “macro” region can bedetermined using a detector that is separate from the detectors 172 a-b.For example, a separate detector can be directed towards the CIS sample190 in order to acquire photoluminescence measurements of the “macro”region alongside the detectors 172 a-b. In some cases, thephotoluminescence of this “macro” region can be determined by one of thedetectors 172 a-b. For example, in some implementation, the optical pathbetween the CIS sample 190 and the detectors 172 a-b can be varied whileimaging the “macro” region, such as by using a different objective lens154 or adjusting the optical properties of the objective lens 154 whileimaging the “macro” region.

The resulting photoluminescence map is examined for variations inintensity indicative of defects typically associated with the siliconsubstrate of a CIS device. For instance, silicon wafers manufactured bythe Czochralski process (i.e., “CZ wafers”) often include curvedvariations in photoluminescence when illuminated by excitation light. Asan example, FIG. 2 shows an example photoluminescence map 200 for a CISdevice. In this example, the photoluminescence map 200 includes severalvariations in photoluminescence intensity 210, appearing as circular orcurved bands of intensity variation. In some cases, variations similarto those shown in FIG. 2 are characteristic of silicon wafersmanufactured by the Czochralski process, and this characteristic patternis not present in the epitaxial layers of the device (e.g., CIS activearea).

The contribution of photoluminescence from the substrate to the totaldetected photoluminescence can be quantitatively calculated byilluminating the CIS sample with long wavelength illumination (e.g.,near infrared illumination with an energy large than approximately 1.1eV, the energy gap of Si), such that a large portion of the carriers(e.g., substantially most or substantially all of the carriers) aregenerated in the substrate. The changes in this photoluminescenceintensity due to the characteristic substrate defects are used as areference for calculations in the substrate photoluminescencecontribution to the total photoluminescence detected for the shortwavelength.

As an example, when the CIS sample is illuminated with excitation lighthaving a relatively long wavelength (e.g., excitation light having awavelength that causes substantially most or substantially all of thecarriers to be generated in the substrate of the CIS sample), the defectcontrast is 50% (e.g., the photoluminescence intensity map includeslocalized variations in intensity of that differ the surroundingintensity by 50%). However, when the CIS sample is illuminated withexcitation light having a relatively shorter wavelength, the defectcontrast is 5% (e.g., the photoluminescence intensity map includeslocalized variations in intensity of that differ the surroundingintensity by 5%). Thus, in this example, one can estimate that thesubstrate contribution to the total photoluminescence for the relativelyshorter wavelength is about 10% (e.g., 5% divided by 50%).

If the substrate contribution to photoluminescence is too large, thenthe measurement conditions can be adjusted (e.g., by reducing thewavelength or light intensity to the excitation light applied to the CISsample) until the defect pattern is reduced or eliminated.

In some cases, the upper injection limit that can be used for thedetection of defects in a CIS sample can be determined, at least inpart, based on the Auger recombination life time, which decreases withincreasing injection level. As an example, for an injection level of1×10¹⁷ cm⁻³, the Auger life time in an example device is 100×10⁻⁶ sec.For a CIS active region with a 5×10⁻⁶ sec effective recombination lifetime from bulk and interfacial recombination, this Auger recombinationwill contribute about 5% to the effective life time.

The sensitivity of the system 100 to defects in the CIS sample depends,at least in part, on the bulk and interfacial recombination lifetimebeing shorter than the Auger recombination life time. In some cases, thewavelength and intensity of the excitation light can be varied in orderto obtain a particular percentage of contribution Auger recombination tothe effective life time. For example, in some implementations, thewavelength and intensity of the excitation light can be varied such thatthe Auger life time of the CIS active region is less than or equal to 5%of the effective recombination life time from bulk and interfacialrecombination of the CIS active region. In some implementation, thisthreshold corresponds to an injection level of approximately 1×10¹⁷ cm⁻³or less. For the CIS active area of approximately 5 μm thickness, insome cases, this will correspond to the absorption of about 1×10¹⁹photons/cm² sec (e.g., corresponding to a power of 100 mW/cm²) in theCIS active region. Although an example threshold is described above,this is merely an illustrative example. In some cases, the wavelengthand intensity of the excitation light can be varied such that the Augerlife time of the CIS active region is less than or equal to some otherpercentage of the effective recombination life time from bulk andinterfacial recombination of the CIS active region (e.g., 1%, 5%, 10%,15%, or any other percentage).

In system 100 (FIG. 1), the sample is illuminated with light nominallynormally to the sample surface. Accordingly, in this geometry, lightreflected from the sample surface is gathered by the system's objectiveand delivered to the detector. More generally, however, otherconfigurations are also possible. For example, referring to FIG. 3, insome embodiments, oblique (rather than normal) illumination may be used.Here, a system 300 includes an illumination assembly 330 arranged toilluminate CIS sample 190 with excitation light incident along anoptical axis 310 that is at a non-normal angle to the sample surface.For instance, optical axis 310 can be at an angle of 45° or more (e.g.,60° or more, 70° or more) relative to the surface normal. Generally, theangle of incidence should be sufficiently large so that little or noneof the light specularly reflected from the sample surface is collectedby objective lens 154.

Illumination assembly 330 includes light sources 132 a and 132 b,filters 136 a and 136 b, and dichroic beam splitter 138. In addition,illumination assembly 330 includes focusing lenses 340 and 342, whichfocus light from the light sources combined by the beam splitter ontosample 190.

System 300 also includes objective lens 154, filter 158, and field lens160 a. Objective lens 154 and field lens 160 a image the surface ofsample 190 onto detector 172 a. The detector is in communication withprocessing module 174.

Like system 100 shown in FIG. 1, objective lens 154 has an optical axisaligned nominally normal to the surface of sample 190.

Due to the relative orientation between the illumination optical axis310 and the imaging optical axis 320, excitation light that isspecularly reflected from the surface of sample 190 is not gathered byobjective lens 154 and is not delivered to detector 172 a. Onlyphotoluminescence from sample 190, scattered light, and stray light isdelivered to detector 172 a. Accordingly, the image formed at detector172 a is a dark field image. Thus, even where filter 158 is not used,the sources of photoluminescence in the wafer appear as bright regionsin an image.

In general, while certain components of system 300 are shown in FIG. 3,other components not shown in the figure may also be included. Forinstance, the system may also include detector 172 b and/or otheroptical elements (e.g., lenses, filters, stops) for delivering light toor imaging light from sample 190.

In some implementations, it is possible to discriminate betweenphotoluminescence from different processes in a wafer where thedifferent processes result in photoluminescence at differentwavelengths. For example, optical filtering may be used to discriminatebetween photoluminescence from crystallographic defects andphotoluminescence from other sources, such as from band-to-bandtransitions in silicon. Photoluminescence from crystallographic defects(e.g., grain or sub-grain boundaries, dislocation clusters, dislocationloops or precipitates, and/or stacking faults) in silicon waferstypically occurs in an energy range from about 0.7 eV to about 0.9 eV.Conversely, photoluminescence from band-to-band transitions in silicontypically occurs at wavelengths greater than about 1 eV. Accordingly, itis possible to differentiate between photoluminescence from these twodifferent sources by blocking light in one of the wavelength ranges fromthe detector. For instance, an optical filter blocking light havingenergy in excess of about 1 eV may be used to detect onlyphotoluminescence from crystallographic defects becausephotoluminescence from band-to-band transitions in silicon have anenergy of about 1.1 eV or higher at room temperature and will be blockedby such an optical filter. More generally, the optical filter may bedesigned to block photon energies corresponding to band-to-bandtransitions in a variety of materials other than or in addition tosilicon, allowing other materials to be similarly studied. In this way,it is possible to use the techniques described above to identify thepresence and location of individual crystallographic defects in a wafer,e.g., a silicon wafer. Such locations will appear as bright spots in animage acquired using a filter to block photoluminescence fromband-to-band transitions.

As an example, filter 158 may include a band pass filter thatsubstantially transmit light having energies from about 0.7 eV to about1 eV, but blocks (e.g., reflects or absorbs) light having wavelengthsbelow about 0.7 eV and/or above about 1.0 eV. A dichroic filter may beused for this purpose. As a result, only light having photon energiesfrom about 0.7 eV to about 1 eV reaches the detector and contribute tothe detected image.

In embodiments where filter 158 is a reflective filter, the filter maybe oriented at an angle relative to the light path so that reflectedlight is not directed back towards the wafer. Filter 158 may beremovable from the light path, for example, manually or automaticallyremovable via an actuable mounting.

Depending on the embodiment, the pass band of filter 158 can vary. Forexample, the pass band may have a full-width half maximum of about 0.3μm (e.g., from about 0.7 μm to about 1.0 μm), or less (e.g., about 0.25μm, about 0.2 μm, about 0.15 μm, about 0.1 μm, about 0.05 μm). The passband may be selected in order to selectively block photoluminescencefrom one source while transmitting photoluminescence from anothersource. For example, narrower pass bands may allow for differentiationbetween different types of crystallographic defects.

While filter 158 is one implementation, others are possible. Forinstance, the filter location is not limited to that depicted. Ingeneral, the filter may be located anywhere in the light path from thewafer to the detector. In some embodiments, the filter may be located atthe detector (e.g., integrated with the detector). In certainembodiments, the filter may be located at a pupil plane of the imagingsystem.

Monitoring wafers for crystallographic defects during different steps inthe wafer processing can allow for the identification of process-inducedcrystallographic defects (such as individual stacking faults,dislocation loops or precipitates). Such defects may be identified withsub-micron resolution. Process steps that may be monitored include ionimplantation steps, annealing steps, layer deposition steps (e.g.,epitaxial layer growth), oxidation steps, and/or polishing steps (e.g.,chemical mechanical polishing). In general, defect characterization maybe performed before and/or after any of these process steps.

Although implementations for detecting defects in CIS devices aredescribed herein, this is merely one illustrative application. Inpractice, implementations can be used to detect defects in other devicesor circuit in which generated minority carriers and photoluminescenceare substantially confined to device active region. For example, in somecases, implementations can be used to detect defects in CMOS circuitsbuilt using fully depleted silicon on insulator (SOI) technologies. Insome embodiments, wafers other than those formed from pure silicon maybe used such as SiGe layers. Compound semiconductor wafers may also beused. For example, wafers formed from III-V or II-VI compounds may becharacterized using the techniques disclosed herein. More generally, theabove techniques may be applied to a variety of materials thatdemonstrate crystallographic defect photoluminescence, and particularlyin those semiconductor materials in which crystallographic defectphotoluminescence and band-to-band photoluminescence occurs at differentwavelengths.

While this specification contains many details, these should not beconstrued as limitations on the scope of what may be claimed, but ratheras descriptions of features specific to particular examples. Certainfeatures that are described in this specification in the context ofseparate implementations can also be combined. Conversely, variousfeatures that are described in the context of a single implementationcan also be implemented in multiple embodiments separately or in anysuitable sub-combination.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the invention. Accordingly, otherimplementations are within the scope of the following claims.

What is claimed is:
 1. A method comprising: illuminating a silicon waferwith excitation light having a wavelength and intensity sufficient toinduce both photoluminescence from band-to-band transitions in thesilicon and photoluminescence from crystallographic defects in thesilicon in the wafer; filtering excitation light reflected from theportion of the wafer and photoluminescence emitted from a portion of thewafer in response to the illumination to provide filteredphotoluminescence, wherein the filtering substantially removes thephotoluminescence from band-to-band transitions in the silicon from thefiltered photoluminescence; directing the filtered photoluminescenceonto a multi-element detector to image the portion of the wafer on thedetector with a spatial resolution of 1 μm×1 μm or smaller; andidentifying one or more crystallographic defects in the silicon in thewafer based on the detected filtered photoluminescence, wherein thecrystallographic defects correspond to bright regions in the image ofthe portion of the wafer.
 2. The method of claim 1, wherein the detectedfiltered photoluminescence comprises light having an energy in a rangefrom about 0.7 eV to about 0.9 eV.
 3. The method of claim 1, wherein thefiltering substantially blocks light having an energy of more than about1.0 eV from being detected.
 4. The method of claim 1, wherein the waferis a wafer for a complementary metal-oxide semiconductor (CMOS) imagingsensor.
 5. The method of claim 4, wherein the portion of the wafercorresponds to one or more pixels of the CMOS imaging sensor.
 6. Themethod of claim 5, wherein identifying one or more defects in the wafercomprises identifying one or more defective pixels of the CMOS imagingsensor.
 7. The method of claim 5, wherein identifying one or moredefects in the wafer comprises identifying one or more defects having adimension of 1 μm or smaller.
 8. The method of claim 1, furthercomprising adjusting a property of the excitation light.
 9. The methodof claim 8, wherein adjusting the property of the excitation lightcomprises adjusting a wavelength of the excitation light.
 10. The methodof claim 9, wherein the wavelength of the excitation light is adjustedto increase photoluminescence emitted from the second portion of thewafer.
 11. The method of claim 10, wherein the second portion of thewafer is at a different depth in the wafer from the first portion. 12.The method of claim 1, wherein the excitation light has a wavelength ina range from 200 nm to 1,100 nm.
 13. The method of claim 1, furthercomprising performing a processing step on the wafer.
 14. The method ofclaim 13, wherein the processing step is selected from the groupconsisting of an ion implantation step, an annealing step, a layerdeposition step, an oxidation step, and a polishing step.
 15. The methodof claim 13, wherein the processing step is performed after identifyingthe crystallographic defects.
 16. The method of claim 15, furthercomprising illuminating the processed wafer with excitation light andidentifying one or more additional defects in the processed wafer basedon photoluminescence from the processed wafer.
 17. The method of claim16, further comprising comparing the crystallographic defects identifiedin the wafer with the additional defects.
 18. The method of claim 1,further comprising identifying crystallographic defects at varyingdepths of the silicon wafer by varying a focal depth of an objectivelens assembly used to image the portion of the wafer on the detector.19. The method of claim 1, wherein the wafer comprises an integratedcircuit.
 20. A system comprising: an illumination module configured toilluminate a silicon wafer with excitation light having a wavelength andintensity sufficient to induce both photoluminescence from band-to-bandtransitions in the silicon and photoluminescence from crystallographicdefects in the silicon in the wafer; a detection module comprising amulti-element detector configured to detect photoluminescence emittedfrom a portion of the wafer in response to the illumination; imagingoptics configured to image the portion of the wafer onto themulti-element detector with a spatial resolution of 1 μm×1 μm orsmaller; an optical filter arranged to block from the detection moduleexcitation light reflected from the wafer and to filterphotoluminescence emitted from the portion of the wafer prior todetection by the detection module to provide filtered photoluminescenceto the detection module, wherein the filtering substantially removes thephotoluminescence from band-to-band transitions in the silicon from thefiltered photoluminescence; and a processing module configured toidentify one or more crystallographic defects in the silicon in thewafer based on the detected filtered photoluminescence, wherein thecrystallographic defects correspond to bright regions in the image ofthe portion of the wafer.
 21. The system of claim 20, wherein theoptical filter transmits light corresponding to the photoluminescencefrom crystallographic defects in the wafer to the detection module. 22.The system of claim 20, wherein the optical filter transmits lighthaving an energy in a range from about 0.7 eV to about 1.0 eV to thedetection module.
 23. The system of claim 22, wherein the optical filtersubstantially blocks light having an energy of more than about 1.0 eVfrom the detection module.
 24. The system of claim 20, wherein theexcitation light has a wavelength in a range from 200 nm to 1,100 nm.25. The system of claim 20, wherein the illumination assembly isarranged to illuminate the excitation light to the wafer along anoptical axis non-normal to an illuminated surface of the wafer.
 26. Thesystem of claim 25, wherein the illumination optics have an optical axisnominally normal to the illuminated surface of the wafer.
 27. The systemof claim 20, wherein the imaging optics comprise an objective lensassembly and the processing module is configured to identifycrystallographic defects at different depths of the wafer depending onthe focal depth of the objective lens assembly.